As a fiber-reinforced composite material in which a carbon fiber, an aramid fiber, a glass fiber, or the like is used as a reinforcing fiber, an isotropic random mat has been used due to formability or convenience of processing. The random mat may be obtained by a spray-up method (dry method) of simultaneously spraying a cut reinforcing fiber body or a thermosetting resin on a mold, a papermaking method (wet method) by adding a cut reinforcing fiber to a slurry containing a binder in advance, or the like.
As a mean for improving mechanical properties of a composite material, increasing a volume content ratio Vf of reinforcing fibers is known, but in the case of a random mat in which cut fibers are used, it is difficult to increase the volume content ratio of reinforcing fibers due to presence of fibers of three-dimensional direction, a lot of entanglements of fibers, and the like. Further, in the case of using the random mat, since the fibers are discontinuous in comparison with a case of using a continuous fiber, it is difficult to sufficiently develop strength of the reinforcing fibers, and a strength development ratio of the reinforcing fibers after forming a shaped product becomes 50% or less with respect to a theoretical value. In Non-patent Document 1, a composite material made from a random mat of a carbon fiber in which a thermosetting resin is used as a matrix is exemplified. The strength development ratio of the composite material is about 44% with respect to the theoretical value.
Further, a composite material in which a thermoplastic resin is used as a matrix in the related art was obtained by heating and pressurizing, by using an autoclave for 2 hours or more, an intermediate material called a prepreg, in which a thermosetting resin is impregnated into a reinforcing fiber base material in advance. Recently, an RTM method where a reinforcing fiber base material into which a thermosetting resin is not impregnated is set in a mold and then the thermosetting resin is cast therein has been proposed. The RTM method achieves that a molding time is largely reduced, but even in the case of using the RTM method, 10 minutes or more is required until one component is molded.
Accordingly, a composite in which the thermoplastic resin is used instead of the thermosetting resin as a matrix has been paid attention.
A thermoplastic stamping molding TP-SMC using a thermoplastic resin as a matrix (Patent Document 1) is a method in which chopped fibers in which the thermoplastic resin is impregnated in advance are heated at a melting point or more, the heated fibers is put into a part of a mold, the mold is immediately tightened, and then the fibers and the resin are allowed to flow in the mold to obtain a product shape and are cooled to mold a shaped product. In this method, the molding may be performed at a short time of about 1 minute by using the fibers into which the resin is impregnated in advance. The method is a method using a molding material called an SMC or a stampable sheet, and in the thermoplastic stamping molding, there are problems in that since the fibers and the resin are allowed to flow in the mold, a thin-walled product may not be made, and since an alignment of fibers are disordered, control thereof is difficult.
Further, in Patent Document 2, fibers in a fiber assembly are continuously distributed in a range of 1 mm to 15 mm of fiber lengths, short fibers are aggregated while being randomly mixed to prevent the fibers from partially forming aggregation-unevenness and to obtain the fiber assembly which is uniform and has excellent isotropy. However, in the method, there is a problem in that the short fibers are oriented in a thickness direction as well.